This invention relates to a material transfer mechanism and particularly to one useful for transporting pallets which support workpieces and for enabling the pallets to be rotated to any desired position while dwelling at a particular work station.
Typical transfer machines intermittently advance pallets along a transfer line to various work stations where operations such as machining, drilling, milling, etc. are performed on workpieces carried by the pallets. Advancement of the pallets along the transfer line is typically achieved by using an elongated transfer bar having a plurality of radially extending dogs which engage the pallets. The transfer bar is rotated to a position wherein the dogs engage the pallets, and is stroked longitudinally to index the pallets to their next position. The transfer bar is thereafter rotated so that the pallet dogs disengage the pallets, and is then retracted to its original position. This process of longitudinal and rotational motion is continued cyclically to index the pallets to each of the work stations in the line. In order to enable machining of the various surfaces of a workpiece, it is often necessary to rotate the workpiece about a vertical axis in order to permit a tool to access the desired workpiece area.
Rotation of the pallets in accordance with the prior art techniques requires a dedicated workpiece turning station which lifts a pallet and rotates it 90 degrees and then lowers it to reengage the pallet supporting rails for continued advancement along the transfer line. An example of this prior art technique is described in U.S. Pat. No. 3,271,840.
Although workpiece turning devices according to the prior art operate statisfactorily in many situations, they have significant limitations. Such mechanisms enable rotation only in 90-degree increments. Rotated positions of the workpiece at other angles is not permitted. If all four facing surfaces of a workpiece require machining, as many as three separate turning devices are needed at dedicated stations to rotate the workpiece through the required 270 degrees of rotation. More significantly, however, the prior art requires that dedicated pallet rotating stations be provided between the stations where machining operations occur, thus lengthening and complicating the transfer line.